A photocoupler is a device achieving both electrical insulation and signal transmission and is widely applied in various fields. In particular, a photocoupler for high-speed digital signal transmission is widely used in Programmable Logic Controller (PLC), Factory Automation (FA) network communication and so on. In a practical use environment, a fine noise component is superimposed on an input waveform in some cases, and for suppression of the malfunction of the output signal due to the noise component, hysteresis for shifting a comparator threshold value according to the input/output level is generally introduced in the design of a comparator threshold voltage. However, the occurrence of a difference in comparator threshold value of pulse signal rising and falling affects the pulse width distortion and transmission delay time, and a problem remains in the viewpoint of reducing the power consumption because of the limitation of the comparator threshold value.